The present invention relates to a proportional or linear solenoid in which a rotation angle of a rotary shaft is determined by a balance of the rotating force of a rotary magnet and the resorting force of a torsion bar.
A prior art proportional solenoid used in various devices comprises, as shown in FIG. 1, a cylindrical case 100 in which a hollow cylindrical coil 101 is mounted and a core 102 is mounted at the center of the coil 101. A plunger 103 is mounted with a constant air gap 104 to the core 102 to form a moving element. Numeral 105 denotes an oil impregnated plastic member located at the center of the core 102 to support the plunger 103. The proportional solenoid has an air passage 106 and is used to control an air flow rate.
In the device of the structure described above, the plunger 103 has a stroke of only approximately 5 mm. When it is to be used in an exhaust gas control apparatus for an automobile, a flow rate control from 0 to 600 l/min. is necessary. Accordingly, the air passage 106 must have a diameter of 14 mm or larger. This results in the increase in an overall apparatus. Furthermore, since the air gap 104 is opened and closed by the linear motion of the plunger 103, it is easily subject to vibration.